This invention relates to a process for preparing protein microspheres having a high protein content. The protein microspheres of this invention are prepared by contacting an aqueous mixture of the protein and a polymer with a surface having a high ratio of surface area to volume. The protein microspheres are suitable for preparing pharmaceutical compositions which can be delivered to a patient, principally by pulmonary, parenteral and oral administration routes. The process can be operated as a continuous process for increased efficiency and productivity.
The preparation and delivery of therapeutic proteins of interest is an area of concentrated research and development activity in the pharmaceutical industry. It is highly desirable to formulate proteins with select release characteristics in the patient with maximum clinical effectiveness. For pulmonary administration, the protein is ideally prepared in the form of discrete microspheres, which are solid or semi-solid particles having a diameter of between 0.5 and 5.0 microns. It is also desirable for the particles to have a protein content as high as possible for maximum therapeutic effectiveness.
Microspheres have been commercially available for biochemical and biotherapeutic applications for many years. For example, antibodies conjugated to beads produce relatively large particles which are specific for a particular ligand. These large antibody-coated particles are used to bind receptors on the surface of a cell for cellular activation, for binding to a solid phase for immunoaffinity purification, and for the delivery of therapeutic agents to a target using tissue or tumor-specific antibodies. The beads can be formed from synthetic polymers or proteins, although synthetic polymers are sometimes preferred due to durability and cost.
Microparticles produced by standard production methods frequently have a wide particle size distribution, lack uniformity, fail to provide adequate release kinetics, and are difficult and expensive to produce. Furthermore, the polymers used to prepare these microspheres are usually soluble in organic solvents, requiring the use of special facilities designed to handle organic solvents. The organic solvents can denature proteins or peptides contained in the microspheres, and may also be toxic when administered to humans or animals.
In addition, the microparticles may be large and tend to form aggregates, requiring a size selection process to remove particles considered to be too large for administration to patients by injection or inhalation. This requires sieving and resulting product loss. Large size particles can also require the use of large gauge needles for injection, often causing discomfort for the patient.
Currently available microspheres are designed to release proteins in an aqueous medium, by incorporating the proteins into a hydrophobic erodible or non-erodible matrix. Many particles exhibit release kinetics based on both erosion and diffusion. In this type of system, an initial burst or rapid release of the drug is observed. This burst effect often results in unwanted side effects in some patients.
U.S. Pat. No. 5,981,719, U.S. Pat. No. 5,849,884 and U.S. Pat. No. 6,090,925, the disclosures of which are incorporated by reference herein in their entirety, describe microparticles formed by combining a macromolecule, such as a protein or peptide, and a polymer in an aqueous solution at a pH near the isolelectric point of the macromolecule. The solution is heated to prepare microparticles having a protein content of greater than 40%. The microparticles thus formed comprise a matrix of substantially homogeneous, intertwined macromolecules and polymers, which permit the aqueous medium to enter and solubilize the components of the microparticle. The microparticles can be designed to exhibit short-term or long-term release kinetics, providing either rapid or sustained release characteristics.
U.S. Pat. No. 6,051,256 relates to processes for preparing powders of biological macromolecules by atomizing liquid solutions of the macromolecules, drying the droplets, and collecting the resulting particles. Biological macromolecules which can be used in this process include insulin and calcitonin.
It will be appreciated that there is a continuing need for a process for preparing and delivering biological agents as microspheres to maximize their effectiveness and minimize the safety concerns for the therapeutic agent.